Organic light emitting diodes (OLEDs) are particularly useful for lighting because they can be fabricated relatively simply and at low cost to cover a large area on a variety of substrates. They are also bright and may be coloured (red, green and blue) or white as desired. OLEDs may be fabricated using either polymers or small molecules: examples of polymer-based OLEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of so-called small molecule based devices are described in U.S. Pat. No. 4,539,507. In this specification references to organic LEDs include organometallic LEDs.
To aid in understanding embodiments of the invention it is helpful to describe an example structure of an OLED device. Thus, referring to FIG. 1a, this shows a vertical cross-section through a portion of an OLED 10 comprising a transparent substrate 12 on which metal, for example copper, tracks 14 are deposited to provide a first electrode connection, in the illustrated example an anode connection. A hole injection layer (HIL) 16 is deposited over the anode electrode tracks, for example a conductive transparent polymer such as PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene). This is followed by a light emitting polymer (LEP) stack 18, for example comprising a PPV (poly(p-phenylenevinylene)-based material. The hole injection layer helps to match the hole energy levels of the LEP stack to those of the anode metal. This is followed by a cathode stack 20, for example comprising a low work function metal such as calcium or barium for the LEP stack and cathode electrode energy levels matching or comprising an electron injection layer such as lithium fluoride, over which is deposited a reflective back electrode, for example of aluminium or silver.
The OLED example of FIG. 1a is a “bottom emitting” device in which light is emitted through the transparent substrate, made for example of glass or plastic. However a “top emitting” device may also be fabricated in which an upper electrode of the device is substantially transparent, for example fabricated from indium tin oxide (ITO) or a thin layer of cathode metal (say less than 100 nm thick).
Referring now to FIG. 1b this shows a view of the OLED device 10 of FIG. 1a looking towards the LEP stack 18 through the substrate 12, that is looking into the light-emitting face of the device through the “bottom” of the device. This view shows that the anode electrode tracks 14 are, in this example, configured as a hexagonal grid or mesh, in order to avoid obscuring too much light emitted by the LEP stack 18. The (anode) electrode tracks 14 are connected to a solid metal busbar 30 which runs substantially all the way around the perimeter of the device, optionally with one or more openings 32, which may be bridged by an electrical conductor to facilitate a connection to the cathode stack of the device.
FIG. 1c shows a Lighting Panel 100 comprising a plurality of OLEDs 10 having a structure as shown in FIGS. 1a and/or 1b. 
Metal tracks, such as the anode tracks 14, are provided in OLEDs such as those of FIGS. 1a-c to increase the conductivity of an electrode and to enable current distribution over a wider area, preferably more uniformly. Thus, the metal tracks 14 preferably have sufficient coverage and conductance to provide a rate and distribution of charge flow that allows the desired amount and uniformity of the luminance of the OLED device. The metal tracks may be placed at intervals of, e.g., tens of μm to a few cm, across the lateral extent of a large area OLED lighting panel. However, deposition of active OLED layers on top of a non-planar surface may result in thickness and/or contour variations, i.e., non-planar surface regions, of the layers. Such variations may for example result in luminance non-uniformities, device instabilities and/or device failure due to electrical shorts (localised regions of higher current densities (‘hot spots’) for example between the tracks and light emissive layer, or in the light emissive layer) in the device. Edges of the metal tracks may cause such thickness and/or contour variations.
Therefore, metal tracks in Lighting Panels comprising OLEDs are preferably planarised prior to processing of the light-emitting and associated (e.g., charge injection) layers. Infill planarisation of the metal tracks may be provided, for example by depositing photoresist or other, generally electrically insulating, planarization materials over the metal tracks.
The anode electrode of an OLED device as described above may comprise an ITO layer on the substrate for current distribution. Such ITO layer typically has a sheet resistance of, e.g., 20-50 Ohms/sq. Use of ITO as anode material may thus be advantageous for providing an electrically conductive anode. However, the sheet resistance of such ITO layer may not be low enough to provide the desired amount and/or uniformity of the luminance of an OLED device. Moreover, the cost of the ITO material and its deposition process are relatively high and this may be significant in relation to products such as, for example, large area Lighting Panels which may comprise a plurality of OLEDs. In addition, the ITO layer has a refractive index of typically ˜1.7-1.9 which is significantly higher than the refractive index (˜1.5) of conventional glass or plastic substrates employed in the manufacture of OLEDs. This mismatch in refractive indices between the substrate and the ITO layer can cause optical losses due to light being trapped in waveguided modes.
Similar considerations apply to other optoelectronic devices, e.g., photovoltaic (PV) devices or other electroluminescent devices.
Therefore, there remains a need to provide an optoelectronic device that can be, inter alia, fabricated relatively simply and/or at low cost and/or has improved performance, preferably not comprising ITO. More specifically, the field of optoelectronic devices continues to provide a need for, e.g., greater efficiency (light to electrical energy conversion or vice versa), improved uniformity of light output or absorption and/or energy conversion across the device, improved reliability and/or lifetime (for example reducing or eliminating the occurrence of electrical shorts), lower cost, etc. Such a need exists for example in relation to devices with relatively large dimensions, e.g., an OLED Lighting Panel. For use in understanding the present invention, the following disclosures are referred to:    Large Area ITO-free Flexible White OLEDs with Orgacon PEDOT:PSS and Printed Metal Shunting Lines, Harkema et al., Proc. SPIE, Vol. 7415, 74150T (2009);    Presentation from Comedd-Opening on Oct. 30, 2008, “Organic Lighting and Organic Solar Cells”, Prof. Dr. Karl Leo, Fraunhofer IPMS, available from http://www.ipms.fraunhofer.de/common/comedd/presentation/leo.pdf;    Osram Datasheet “ORBEOS™ for OLED Lighting”, dated 2009 Nov. 18, available at least from May 18, 2010, from http://www.osram-os.com/osram—os/EN/Products/Product_Promotions/OLED_Lighting/Technical_Information/index.html;    International patent application publication WO2004/068389, Conductive Inkjet Technology Ltd., et al., inventors Hudd et al., published Aug. 12, 2004;    Korean publication KR2008004919, published 2008 May 14, Samsum Electronics Co Ltd.;    Japanese publication JP2007242829, published 2007 Sep. 20, Rohm Co Ltd.;    Japanese publication JP5094880, published 1993 Apr. 16, NEC Corp.;    United States patent application publication US2007/0126348, published Jun. 7, 2007, Iou, and CN1832647, AU Optronics Corp.;    Korean publication KR20040040242, published 2004 May 12, LG Philips LCD Co Ltd.;    International patent application publication WO00/36662, published Jun. 22, 2000, Cambridge Display Technology Ltd.;    Low-cost, large area production of flexible OLEDs a step closer, Press Release Apr. 7, 2009, Agfa Materials and Holst Centre; and    Highly-efficient OLEDs on ITO-free polymeric substrates, Fehse et al., Proc. SPIE Vol. 6192, 61921Z, 2006.
For general background information relating to OLEDs, information on device structures and methods of making OLED devices are described in the book “Organic Light-Emitting Materials and Devices”, edited by Zhigang Li and Hong Meng, published by CRC Press (Taylor and Francis) in 2007 (ISBN 1-57444-574-X), especially Chapters 2 and 8 for polymer materials and devices.